Adsorption cooling technology is an established alternative to mechanical vapor compression refrigeration systems. Adsorption-based refrigeration systems have several advantages including few moving parts, reduced vibration and lubrication requirements, and scalability to refrigeration load from a few watts to several kilowatts. Key disadvantages include large thermal mass, bulkiness, complex controls, and low COP (0.2 to 0.6). These disadvantages arise mostly from thermophysical property limitations of adsorbent-refrigerant combinations presently available.
In the United States, various types of chiller technologies deployed in commercial buildings consume approximately 0.6 quads annually. Adsorption chillers, while currently a relatively small percentage of the chiller market, represent an important alternative to mechanical vapor compression refrigeration systems. Adsorption chillers use low-grade heat to drive a thermal compressor. Low-grade industrial heat sources range from waste heat produced in manufacturing to solar thermal installations. Adsorption-based cooling systems have several advantages: using low-grade heat sources; include few moving parts; reduced vibration and lubrication requirements; and scalability of cooling load from a few watts to several kilowatts. Key disadvantages of a typical adsorption chiller system include large thermal mass, bulkiness, and low COP (0.2 to 0.6). The low COP is the most significant factor that drives up the size and cost of the units per ton of cooling capacity delivered and thus is a significant barrier to more widespread use of this technology.
A basic adsorption chiller machine consist of two sorbent chambers, one evaporator, and one condenser. While the sorbent in the first compartment (desorber) is regenerated using heat from the external heat source, such as from hot water, the sorbent in the second compartment (adsorber) adsorbs the vapor of the working fluid entering from the evaporator; this compartment has to be cooled in order to enable a continuous adsorption. The working fluid is evaporated in the evaporator, and through heat exchange is used to cool or chill an external heat transfer fluid, typically water (e.g. chilled water). When the sorbent in the adsorber becomes saturated with working fluid, the function of the compartments is switched. Exemplary, non-limiting, schematics of basic adsorption chiller systems are shown in FIGS. 1 and 2. In a basic configuration, a hot fluid, such as hot water, is used to heat the desorber to regenerate the sorbent; a cool fluid, such as cool water, is used to cool the sorbent in the adsorber; a cool fluid is used to cool the condenser, optionally being the same fluid or fluid stream as used to cool the adsorber; chilled fluid, such as chilled water is cooled in the evaporator. Various configurations of heat exchangers or heat transfer configurations can be used in the evaporator, condenser, adsorber, and desorber. Various control and engineering schemes may be used to switch the functions of the two sorbent chambers. In a non-limiting example, the connection between the sorbent chambers and the evaporator and condenser may include flapper values; when chamber 1 is the desorber and chamber 2 is the adsorber (as in FIGS. 1 and 2) the flapper valve(s) connecting chamber 1 to the evaporator is(are) closed while the flapper valve(s) connecting champer 1 to the condenser is(are) open; and the flapper valve(s) connecting chamber 2 to the evaporator is(are) open while the flapper valve(s) connecting champer 2 to the condenser is(are) closed; when switching the functions of the two sorbent chambers the open/closed positions of the flappers values are switched. In a non-limiting example, when the functions of the two sorbent chambers are switched the circuits for the hot fluid and cool fluid may also be switched, such as through the use of controlled valves. The system may also consist of a circuit to return liquid working fluid condensed in the condenser back to the evapor.
The present invention is directed toward adsorption systems having very high mass loading and relatively low heats of adsorption comprising certain combinations of refrigerants and sorbents such as metal-organic framework (MOF) which provide highly efficient adsorption chillers.